1. Field of the Invention
The invention relates to electric power steering systems for vehicles.
2. Description of the Related Art
Traditionally, power steering systems have utilized hydraulics to provide assist to the driver of the vehicle. These hydraulic power steering systems typically include a hydraulic pump operatively connected to and driven by the vehicle engine. The hydraulic pump is in fluidic communication with a cylinder operatively connected to the steering column. One drawback to hydraulic power steering systems is that the hydraulic pump is always being powered by the engine, thus affecting overall fuel economy of the vehicle.
As such, electric power steering (EPS) systems have been developed and introduced into vehicles. A typical EPS system utilizes an electric motor operatively connected to the steering column or steering gear assembly to provide the assist to the driver. The electric motor is typically controlled by a controller including power circuitry (e.g., power MOSFETs). The motor assist torque is controlled by the controller based on a variety of vehicle and system inputs. These inputs may include vehicle speed, handwheel torque, steering angle, etc. Additionally, the motor toque may be controlled as a function of system voltage, motor position, and motor velocity. Since motor velocity is a resultant of the controlled motor torque and the vehicle operation condition, the motor velocity is not directly controlled. Therefore, the motor velocity needs to be measured if it is used as part of the overall control strategy. Unfortunately, velocity measurement errors (due to accuracy or signal latency), may lead to an incorrect amount of electrical current being applied to the motor. Furthermore, direct measurement of velocity can be cost prohibitive. Therefore, other velocity measurement techniques are often used to calculate (or estimate) the motor velocity.
The invention of the subject application solves this and other problems.